This invention concerns a method of directly measuring a physical property of a fluid such as gas density and/or using the measured physical property to determine another property such as gas pressure. Conventionally, gas density can be determined by weighing a gas container, measuring its volume and subtracting the container's empty weight. The weight difference is then divided by the container's volume to obtain the density. A major difficulty resides in trying to measure the filled container's weight, especially when the container is mounted for use. Gas density can also be determined by applying gas law data to a measured pressure. This requires an accurate pressure and temperature measurement, and an accurate determination of the gas law for the measured gas. The gas law in turn will vary from gas to gas and at different combinations of pressure, density and temperature.
Gas pressure measurement also presents challenges. At the present time, one of the most common methods of gas pressure measurement is the Bourdon Tube. This simply consists of a bent or coiled tube closed at one end with the other end open and mechanically fixed to the system whose gas pressure is to be measured. The outside of the tube is kept at a reference pressure (usually atmosphere). When the pressure inside the tube exceeds the pressure outside the tube, the tube will began to straighten. When the pressure inside falls, the tube will began to return to its bent form. Typically, a device such as a hinged meter needle, is attached to the closed end of the Bourdon tube. As the tube moves the needle then moves along the meter scale. By appropriate choice of materials and tube dimensions, it is possible to achieve a meter indication that is approximately linear or at least smoothly varying with respect to pressure variation. The disadvantages of the Bourdon tube reside in the difficulty of repeatably reproducing the same mechanical behavior from tube to tube. This results in a need to either mechanically calibrate each tube or accept a large variation in pressure response from tube to tube. The motion of the tube is also susceptible to temperature variation. As the tube heats its thermal expansion will cause extension and its other mechanical properties will also vary. Additionally, a mechanical linkage is required to whatever is used as a meter. Mechanical linkages necessarily exhibit hysteresis, wear and `play`.
Another common measuring technique is to place a strain gauge on a diaphragm. Strain gauges can be composed of materials such resistive films or piezoelectric elements, among others. The diaphragm in turn is fixed at the edges and is placed between the pressure to be measured and a reference pressure. As the pressure difference between the measured and reference sides of the diaphragm varies, the diaphragm will flex towards or away from the reference pressure. This flexing results in strain applied to the strain gauge which in turn provides an electrically measurable indication of the degree of strain. The degree of strain is then hopefully proportional to the pressure difference. Strain gauge pressure measurement has the advantage of providing a direct electrical signal that can be readily monitored by automatic equipment such as computers or controllers. Strain gauge meters can also be made much smaller than Bourdon tube devices and can even be micromachined into integrated circuit wafers. Disadvantages include temperature induced variation in response, poor unit to unit repeatability and high cost.
Optek Technology, Inc. of West Crosby Road, Carrollton, Tex. 75006 has a published data book (copyright 1989, 1990) that indicates its OPB model XXX series sensors can be used to sense the absence or presence of a liquid. FIG. 5 of Optek Application Bulletin 204 (July 1989) notes that light signal variations due to reflection at an interface between a liquid and a transparent material can be used to detect the presence of a liquid.
In laboratory settings it is possible to indirectly determine gas density or pressure by monitoring variations in the speed of sound or speed of light in the gas. However, until now this has not translated into a low cost, practical and readily employable device outside the lab.